1. You are correct. It will collect all the predefined fields you are seeing blank at the moment such as System name, MAC address ect..
2. An IP will only be queried via SNMP if it responds first to a Ping. Secondly within the IPAM credentials you have the ability to specify the priority order in which the SNMP communities will be used. To avoid needless retries make sure the most common communities are at the top. Lastly the default subnet scan time in IPAM is 4 hours so the over head for the application and/or devices should not be a concern and you can increase this interval if you wish.
Hope this helps!